I started to edit this to improve the question and the formatting and then ran into the word "preposition". Which preposition? Please edit the question to detail exactly what you are asking about (re-quote words as necessary).
– Andrew Leach♦Jan 25 '13 at 10:33

@Andrew: I think this is essentially asking the difference between "learned" and "was learning". Also, we should point out that without two blank spaces at the end of a line, the text will be displayed on the same line – something that can take awhile to figure out.
– J.R.Jan 25 '13 at 10:36

@AndrewLeach I guess the question is which aspect to pick when there's a reference time span with "for" — in this case, "for five years". I also think this could be a duplicate.
– RegDwigнt♦Jan 25 '13 at 10:38

I've found plenty about past progressive vs simple past, but none yet on how they relate to a preposition detailing the period. I'll add an answer, but I wouldn't be surprised if there is indeed a duplicate.
– Jon HannaJan 25 '13 at 10:39

1

If your purpose for asking this question is to most clearly state that you studied piano for 5 years, I would say, "I studied piano for 5 years. "learned" is awkward in either sentence.
– Kristina LopezJan 25 '13 at 12:00

1 Answer
1

The first is in the past progressive tense, the second in the simple past tense.

Let's consider if the preposition was absent, first:

I was learning to play the piano

This means that during the period of time referred to, your learning to play the piano was an ongoing event.

I learned to play the piano

This means that at some point in the past, you learned to play. It may have taken a long period of time (and of course, likely did), or may have happened in a flash (unlikely in real-life, but the grammatical construct covers a sci-fi "I know kung fu!" method of learning just as well as it does five years of study, not least because it would also apply to things one can indeed learn in a matter of minutes).

Now, when we add the time using for, the meaning of the two moves closer together. With the progressive we now have the period of that continuous time detailed. With the simple, we now have a continuous period defined - giving the entire sentence some of the qualities of a progressive past construct.

They still differ though. There are times when we can certainly use either, but the progressive remains more appropriate for talking about things that happened during that period, and the simple for identifying that period as a single, if prolonged, point in the past:

I was learning to play the piano for five years. I didn't have much time for socialising.

The lack of free time is during the period described by the progressive.

I learned to play the piano for five years. I haven't so much as sat at one since.

I learned to play the piano for five years. Then I went to Julliard.

Note that while the first simple-past example above suggests that after the five years ended there was no more learning piano, the second suggests the opposite; presumably the speaker continued to learn piano in such a prestigious music school, but this implied period of learning the piano is clearly after the five years described in the first sentence.

So even though the preposition introducing a period of time makes the bare meaning of each sentence more similar than before by making the simple refer to a continuous period much as the progressive always does, there is still a difference in how the progressive places the mental focus in that continuous period, and the simple past places it just after.